Printing plates designed such that a silver image obtained by a silver complex diffusion transfer process which is one of photographic reproduction processes is used as an offset printing master are known in the art as disclosed in Japanese Patent Publication (JP-B) Nos. 43132/1971 and 30562/1973. Further, positive type printing masters are disclosed in Japanese Patent Application Kokai (JP-A) No. 55402/1974 and negative ones disclosed in JP-A 106902/1977 and 112402/1977.
Typically the silver complex diffusion transfer process adapted for the preparation of lithographic printing plates is carried out by imagewise exposing a photosensitive material comprising an undercoat layer also serving for anti-halation purpose, a silver halide emulsion layer, and a physical development nucleus layer on a support, and developing the material whereupon the silver halide having a latent image formed therein is converted into black silver in the emulsion layer. At the same time, the silver halide having no latent image formed is dissolved by the action of a silver halide complexing agent in the developer and diffused to the surface of the photosensitive material. The thus dissolved and diffused silver complex salt precipitates on physical development nuclei of the surface layer due to the reducing action of the developing agent, forming a silver image. To enhance the ink receptivity of the silver image, the development step is optionally followed by sensitization. Then the plate is set in an offset press for receiving ink and transmitting an ink image to an ink-receptive sheet.
These printing plates should meet a set of properties which depend not only on a particular type of ink-receptive sheet, but also largely on their preparation process, especially a development step. That is, one of key factors dictating printing characteristics is the state of a transfer-developed silver image, which is, in turn, dictated by conditions of forming transfer silver grains, for example, the diffusion rate, stability and reduction rate of the silver complex salt.
A number of silver halide complexing agents are known for use in the development step of diffusion transfer process. Examples include (1) thiosulfate salts, (2) thiocyanate salts, (3) aminothiosulfates as disclosed in U.S. Pat. No. 3,169,992, (4) cyclic imide compounds as disclosed in U.S. Pat. No. 2,857,276, (5) 2-mercaptobenzoic acids as disclosed in JP-B 4099/1986, and (6) thiosalicylic acids and cyclic imide compounds having a five or six-membered ring as disclosed in JP-B 4100/1986. Printing plates which are prepared through development with such silver halide complexing agents have the advantages of high sensitivity, high resolution, and high image reproducibility, but are prone to plate wear as compared with other practical printing plates such as presensitized plates. It is thus desired to improve the plate wear of such plating plates.
Among the above-mentioned silver halide complexing agents, thiocyanates can be used to form a well ink-receptive silver image with the concomitant benefit of a reduced cost. This procedure, however, has several problems including (1) corrosion in that thiocyanates are metal corrosive and thus tend to attack parts of the processing equipment, (2) restricted solution disposal in that thiocyanate solution is classified as cyan-containing solution according to the waste water regulation and must be deliberately treated before discarding to the environment, (3) cumbersomeness in that thiocyanates are deliquescent so that their handling is difficult and their processing solution requires care during preparation, (4) careful management of processing solution in that ink-receptivity largely depends on the amount of thiocyanate added.